British Prime Minister David Cameron has described Afghanistan and Nigeria as "fantastically corrupt."

It was not clear whether Cameron knew he was being filmed and recorded when he made the comments at a London reception on May 10.

"We've got some leaders of some fantastically corrupt countries coming to Britain -- Nigeria and Afghanistan, possibly the two most corrupt countries in the world," Cameron said, referring to an upcoming anticorruption summit.

The May 12 event in London is to bring together world leaders, business, and civil society to "agree a package of practical steps" to tackle corruption.

"Because it is an anticorruption summit, everything has to be open, so there are no sorts of closed-door sessions -- it's all in front of the press,” Cameron said on May 10. “It's going to be... It could be quite interesting."

Afghanistan is at number 166, second-from-bottom, in Transparency International's latest Corruption Perceptions Index, an annual ranking of countries. Nigeria is ranked 136th.